1. Field of the Invention
The present invention relates to an image pickup apparatus for converting incident light from a desired field to an electric signal to take the image of the field, and an image shooting method using the same. The present invention is advantageously applicable to an electronic still camera, image inputting unit, movie camera, cellular phone or similar imaging apparatus using photosensitive cells with a relatively higher optical sensitivity characteristic and photosensitive cells with a relatively lower optical sensitivity characteristic.
2. Description of the Background Art
Japanese patent laid-open publication No. 298175/1992, for example, discloses a solid-state image pickup apparatus including an image sensor in which photosensitive cells with a relatively lower optical sensitivity, or photoelectric conversion sensitivity, characteristic and photosensitive cells with a relatively higher photoelectric conversion sensitivity characteristic are arranged for enhancing the dynamic range of the apparatus. More specifically, the image sensor has photosensitive cells, or pixels, having a relatively larger photosensitive area each and other photosensitive cells having a relatively smaller photosensitive area each arranged in horizontal lines alternating with each other. For example, the larger and smaller photosensitive cells are arranged on odd- and even-numbered lines, respectively. When bright light is incident on the image sensor, the apparatus clips the outputs of the larger photosensitive cells above a preselected level and then adds the outputs of the smaller photosensitive cells to the outputs of the larger photosensitive cells.
The image pickup apparatus taught in Japanese patent laid-open publication mentioned above has the following problems left unsolved. First, the layout of the photosensitive cells, constituting the image sensor, is strictly limited because the size of the photosensitive cells is determined in dependent upon the lines in which they are involved in order to allow signal charges stored in the two kinds of photosensitive cells to be read out independently of each other. Second, adding the outputs of the larger and smaller photosensitive cells, which are arranged on a respective line each, means obtaining one line of information from two lines of information, so that image resolution available with the image pickup apparatus is substantially halved. Third, because the outputs of the larger or high-sensitivity photosensitive cells are clipped above a preselected level when the outputs of the larger and smaller photosensitive cells are added, the S/N (Signal-to-Noise) ratio of the high-sensitivity cells is reduced.
Moreover, the document stated above does not even suggest how it thins out the outputs of the photosensitive cells in a movie mode.